The treatment of HIV infection has been radically altered by the use of combination anti-retroviral chemotherapy including protease inhibitors. These agents in combination with reverse transcriptase inhibitors produce profound declines in plasma viral RNA, increases in CD/4 counts and improved clinical outcomes. This therapy provides the prospect for long term survival and even disease eradication. At the same time, if these medications are taken in inadequate amount or if the regimen dosing is irregular, the therapeutic benefit is lost and antiviral resistance develops. Protease inhibitors require rigorous dosing schedules, most commonly added to already complex medications regimens. Furthermore, these medications cause trouble side effects and have substantial drug interactions. The patients taking these medications often are functionally compromised with poor health status and may be cognitively impaired. These factors makes the successful treatment of HIV disease less an issue of the availability of potent pharmacological therapeutics and more dependent on patient ability to adhere to a complex medication regiment over time. However, rigorous methods of measuring adherence to anti-retroviral medication are not widely available and most clinical trials have employed minimal, if any, evaluation of medication adherence. The Medication Adherence Core is validating interview-based methodology to measure adherence based on newly developed items and work done in the ACTG and HCSUS studies. These instruments measure patient adherence to work with HIV clinical trials and cohort studies at UCLA to (1) identify opportunities to measure medication adherence, (2) provide and customize instruments for incorporation into existing and planned studies. (3) assist in design of data collection and analysis strategies to measure adherence and factors associated with adherence, and (4) adapt previously validated instruments for new populations (such as pediatrics) and clinical settings. In addition, the Medication Adherence Core will work with clinical trial investigators to develop interventions to enhance adherence and to measure the effects of these interventions. In addition, with the Behavioral Adherence Recruitment and Retention Core, this Core will create coherent assessment modules of medical and behavioral adherence for UCLA investigators. Both Cores will constitute the Health Behavior and Outreach Program of the CFAR.